Recently, there has been strongly desired a high-power semiconductor laser capable of outputting a light power of 30 mW or more as a light source for writing to magneto-optical disks.
Such a semiconductor laser for magneto-optical disks needs to meet the following characteristic requirements:
(1) a single transverse mode oscillation in up to a high-power operation; PA1 (2) small astigmatism (difference of beam waist position between the direction perpendicular to the heterojunction face and the direction parallel to the heterojunction face); PA1 (3) low ellipticity; PA1 (4) low noise; and PA1 (5) high reliability.
In attempt to meet the requirements (1) and (2), there has been proposed a self-aligned semiconductor laser having a waveguide mechanism resulting from complex refractive index, which laser is of double heterojunction structure or MQW structure. The refractive index guiding structure of such a semiconductor laser is realized by a film thickness design such as to form a thin active layer or to make small the distance between an active layer and a current-blocking layer (refer to Ryoichi Ito et al., "Semiconductor Laser", Baifukan, 1989).
On the other side, it is known that the requirements of low ellipticity (3) and high reliability (5) as well as the requirement of small astigmatism (2) are met if there is provided in a clad layer adjacent the current-blocking layer a beam-expanding AlGaAs layer having an Al content lower than that of the clad layer (refer to Nakatsuka, "Reliability of 780 nm High-Power Laser Diodes with Quantum Well Active Layer", Japanese Journal of Applied Physics, Vol. 30, No. 3, pp. 493 to 498, March 1991).
With a semiconductor laser of the refractive index guiding structure, however, a single longitudinal mode oscillation is likely even in the low power operation at a light power of, for example, 3 mW, thus resulting in an increased coherence. It is known that such a semiconductor laser tends to produce returning-light-induced noise and does not meet the above requirement of low noise (4). Further, despite the merit of small astigmatism, the semiconductor laser provided with the beam-expanding layer tends to generate single longitudinal mode oscillation.
Attempts have been made to reduce such noise by superposing high frequency so as to turn the single longitudinal oscillation into multi-longitudinal mode oscillation (refer to D. Welford and A. Mooradian, "Observation of linewidth broadening in GaAlAs diode lasers due to electron number fluctuations", Appl. Phys. Lett., Vol. 40, p. 560, 1982). However, if the longitudinal mode oscilllation before the superposition of high frequency is strong in singleness, such superposition cannot lower a noise level sufficiently.
Alternatively, there is known a method employing a gain guiding structure (refer to Ryoichi Ito et al., "Semiconductor Laser", Baifukan, 1989) as means for realizing multi-longitudinal mode oscillation. However, a semiconductor laser of the gain guiding structure involves a problem of difficulty in maintaining single transverse mode oscillation in up to a high-power operation and, furthermore, suffers from an increased astigmatism.